1. Field of the Invention
This invention relates to a microwave converter, more particularly, to improvements in a circuit configuration for developing a small-sized and compact microwave converter converter for satellite broadcasting.
2. Description of the Prior Art
Recently, satellite broadcasting by a microwave using a broadcasting satellite has been put into practice, and microwave devices which have been used mainly in the industrial field are entering the consumers' field. One example of one such device is a microwave converter for receiving satellite broadcasting. Such a microwave converter has a function as converting a received signal of a UHF band to an IF signal.
Satellite broadcasting is aimed to be received by homes so that a large-sized antenna having a large diameter and used in a satellite communication system is not necessary. In satellite broadcasting, therefore, a large amount of power is used for transmission when compared with that used in satellite communication; This enables the use of a small-sized receiving antenna. However, in order to receive the signals readily at homes with little expense, a satellite broadcasting reception system which employs a smaller antenna than currently used had to be developed. In attempting to solve this problem, a highly efficient antenna, as well as an excellent microwave converter used in connection thereto, have been extensively studied.
With the development of a small-sized antenna, a small-sized and compact microwave converter to be associated with such an antenna had to be developed. Generally, a microwave converter is connected to a primary radiator disposed at a radio focal point of the antenna aperture surface. The primary radiator and microwave converter are supported by a support member extending from an antenna pedestal. When used outdoors, the primary radiator and microwave converter may vibrate because of wind, rain or other elements of nature, causing a shift from the focal point, thereby lowering the receiving efficiency. In order to prevent this problem from arising, the support member should be designed to have sufficient strength and durability. It is necessary to design such a support member to be simple, lightweight and inexpensive, so that the support member is adaptable to a small-sized antenna. A microwave converter of the prior art is considerably bulky and heavy. If such a microwave converter is used, therefore, it is very difficult to solve the above-mentioned problem. Thus, an effective means for solving such a problem is to develop a small-sized and compact microwave converter. It will be appreciated that the realization of a small-sized and compact microwave converter is also effective with respect to the development of a simple, light, handy and inexpensive antenna system even when a conventional antenna is used.
A microwave converter consists mainly of a low noise amplifier, filter, mixer, local oscillator, IF amplifier, power supply, etc. These circuit components except the IF amplifier and power supply are formed by MIC (Microwave Integrated Circuits) which are mainly constructed by microstrip circuits. A significant factor for developing a small-sized and compact microwave converter lies in arranging the circuit components effectively.
Microwave converters of the prior art will be described in more detail with reference to the drawings. FIG. 10 shows a view illustrating schematically the internal arrangement of a prior art microwave converter. In the microwave converter, a microwave signal is transmitted from a waveguide coaxial converter 1 to a low noise amplifier 2 through a microwave signal input terminal 7, and further to a mixer 4 through a filter 3. In the mixer 4, the microwave signal is then mixed with a local oscillating signal from a local oscillator 5, thereby converting the signal at the microwave frequency to an IF signal. The IF signal is sent to an IF amplifier 6, and the output signal of the IF amplifier 6 is then supplied via an output terminal 8 to a demodulator (not shown). A power supply 9 supplies a bias voltage to each circuit. The low noise amplifier 2, filter 3, local oscillator 5, etc. are formed on a dielectric board 10 using mainly microstrip circuits. The circuit components may be assembled integrally on one board or substrate, or they may be assembled individually on a plurality of boards which are then assembled on a base. Since frequencies of the IF amplifier 6 and power supply 9 are relatively low, they may be formed on a separate board. In the prior art example shown in FIG. 10, the main circuit components are arranged on a flat surface.
FIGS. 11A and 11B show another example of a conventional microwave converter. In the figures, similar reference numerals of FIG. 10 indicate similar circuit components. In the microwave converter of FIGS. 11A and 11B, the circuit components are arranged on two flat surfaces. Each of the main circuit components is formed individually on a board. Some of the components are mounted on one surface of a base 11, and the other components on the opposite surface of the base 11. The base 11 is usually made of a metal plate. The microwave converter of FIGS. 11A and 11B operates in the same manner as that of FIG. 10, and, therefore, its detailed explanation is omitted. The circuit components 2, 3 and 4 arranged on the upper surface of the base 11 may be integrally arranged on one dielectric board. Alternatively, the circuit components 2, 3 and 4 and the IF amplifier 6 may be integrally arranged on one dielectric board.
The local oscillator 5 shown in FIGS. 10 and 11B has an oscillation resonator which is usually a dielectric resonator of the electromagnetic field coupling type. Hence, the local oscillator 5 is enclosed in a metal shield case so that the deterioration of the properties of the oscillator, the fluctuation in oscillation frequency caused by external disturbances, and unwanted radiation due to the local oscillator output may be prevented. Consequently, the local oscillator 5 is constructed in a bulky structure which is large in height as compared to the other circuit components used in the microwave converter.
In the examples of the prior art described above, the following problems are encountered with respect to the development of a small-sized microwave converter.
As shown in FIGS. 10, 11A and 11B, the circuit components constituting a microwave converter are arranged flat so that the overall size of the microwave converter tends to become larger lengthwise and breadthwise. Moreover, the local oscillator 5 protrudes largely because it must be enclosed in a metal shield case. The other circuit components can be formed with the height lower than the height of the metal shield case so that the height of a microwave converter depends upon the height of the metal shield case, resulting in that the overall dimensions of the converter become large, particularly in height. It has been desired to develop a novel circuit configuration which can solve the aforementioned problems so as to construct a smaller microwave converter.